1. Field of the Invention
This invention relates to an exhaust gas treating catalyst and an exhaust gas purification apparatus using it. More particularly, the invention relates to an exhaust gas treating catalyst which is effective when used in removing nitrogen oxides, hydrocarbons, and carbon monoxide in an exhaust gas discharged from a diesel engine or the like; and an exhaust gas purification apparatus using the exhaust gas treating catalyst.
2. Description of the Related Art
Exhaust gas treating catalysts for removing nitrogen oxides in an exhaust gas discharged from a diesel engine or the like include, for example, those having a carrier coated on a honeycomb-shaped substrate, the carrier supporting a precious metal and an occluding or storage material for occluding or storing NOx. Examples of the precious metal are platinum, rhodium, and palladium. Examples of the storage material are alkali metals such as potassium, and alkaline earth metals such as barium. Examples of the carrier are alumina, ceria, zirconia, and titania.
Nitrogen oxides in the exhaust gas are removed by installing such an exhaust gas treating catalyst in an exhaust pipe of a diesel engine or the like. That is, when the exhaust gas is in a lean state (a state where the oxygen concentration is high (3% or more)), NO and oxygen react on the precious metal to form NOx, and this NOx is occluded or stored in the storage material.
Another practice is to spray the exhaust gas treating catalyst with an organic compound or a fuel, which will serve as a reducing agent, or increase the injection volume of the fuel in the engine, thereby creating a reducing atmosphere with the exhaust gas in a rich state (a state where the oxygen concentration is low). By so doing, NOx stored in the storage material migrates onto the precious metal. This NOx, hydrocarbons and CO react to generate water, nitrogen and carbon dioxide, and these substances are discharged. With the exhaust gas treating catalyst, therefore, the storage of nitrogen oxides and the discharge of the stored nitrogen oxides as nitrogen (regeneration of the catalyst) are performed repeatedly.
However, the basic metal (alkali metal, alkaline earth metal) as the above-mentioned storage material is poisoned by the sulfur content in the fuel to decrease in storage performance for the nitrogen oxides. The sulfur-poisoned storage material is exposed to a reducing atmosphere at a high temperature, say, a temperature of 650° C. or higher, to desorb and remove sulfur. The reducing atmosphere can be created, for example, by the injection of fuel or the elevation of the exhaust gas temperature, but such an operation worsens fuel economy. Also, the exposure of the catalyst to the high temperature causes heat deterioration. Moreover, the active metal is sintered, so that the performance of the exhaust gas treating catalyst degrades. Furthermore, if a diesel particulate filter (DPF) for removing particulate matter (PM) contained in the exhaust gas is installed in the exhaust pipe, PM trapped by the DPF is burned off, thereby increasing the chances of the exhaust gas treating catalyst being exposed to high temperatures.
Among denitrification catalysts such as NOx storage catalysts, various perovskite-type catalysts, namely, so-called complex oxide-containing exhaust gas treating catalysts, have been developed as the catalysts having high resistance to sulfur and high resistance to heat (see, for example, Patent Document 1).